1. Field of the Invention
This invention relates to a semi-conductor wafer printing apparatus, and more particularly to an apparatus in which a circuit pattern lying on a reticle is positionally-adjusted onto a wafer and exposed to light.
2. Description of the Prior Art
The recent tendency of to make wafers of larger size and to make circuit patterns of finer design has created various problems which have not been salient in the conventional projection exposure system.
Such problems include, for example, the problem of thermal expansion or contraction of the wafer resulting from the ambient temperature, the problem of strain of the wafer during the semiconductor manufacturing process, and the minute deformation of the transferred pattern caused the characteristics of the exposure apparatus used. These problems have become serious as the circuit pattern has been made more minute for the purpose of higher integration of the electronic circuit. Among various wafer exposure methods, the step-and-repeat method is suitable to solve the abovenoted problems. This method has the following advantages:
(1) Adoption of a reduction imaging system makes it easier to design a lens of high resolving power.
(2) Each exposure is carried out on a small section of a wafer. This can remarkably alleviate the influence of the minute deformation of the wafer.
(3) The use of a reduction system as the projection lens makes it less susceptible to the influence of dust on the reticle side.
The system of the stepper may generally be classified into the off-axis method and the TTL (through the lens) method. The off-axis method has an alignment microscope whose position relative to the projection lens is known. This microscope used to position the wafer. The wafer, after positioned, is accurately transferred by a predetermined amount and that state of the wafer is regarded as the standard state. This method is indirect alignment method. Therefore, it has the disadvantages that it is liable to cause errors and that it is difficult to cope with the random component (random localization) of the deformation of the wafer.
On the other hand, the TTL method is a method in which the aligned condition of the reticle and the wafer is observed through an imaging lens directly. The TTL method is superior to the off-axis method in that there is no necessity of providing a reference position in addition to the imaging lens as is done in the off-axis method.
A great advantage of the stepper is that alignment is carried out and optimized with respect to each section of the wafer. This procedure increases the accuracy of alignment. The TTL method also is meritorious in that the aligned condition can be confirmed for each shot through the imaging lens, and this leads to the possibility of ensuring total alignment accuracy.
In the TTL method, however, chromatic aberration exists in the projection lens. It is therefore only in the printing wavelength that the reticle is focused to the wafer. In the other wavelengths, the reticle is out of focus to the wafer. To enable the reticle to be focused to the wafer in the other wavelengths, it is known to change the part of the projection lens or to insert a lens additionally. These methods are meritorious in that the use of a light which does not sensitize photoresist can be permitted, while they are disadvantageous in that mechanical errors arising from the use of additional optics may again cause an alignment error.